matrix-spec/proposals/1219-storing-megolm-keys-serverside.md

Storing megolm keys serverside

Background

A user who uses end-to-end encryption will usually have many inbound group session keys. Users who log into new devices and want to read old messages will need a convenient way to transfer the session keys from one device to another. While users can currently export their keys from one device and import them to another, this is involves several steps and may be cumbersome for many users. Users can also share keys from one device to another, but this has several limitations, such as the fact that key shares only share one key at a time, and require another logged-in device to be active.

To help resolve this, we optionally let clients store an encrypted copy of their megolm inbound session keys on the homeserver. Clients can keep the backup up to date, so that users will always have the keys needed to decrypt their conversations. The backup could be used not just for new logins, but also to support clients with limited local storage for keys (clients can store old keys to the backup, and remove their local copy, retrieving the key from the backup when needed).

To recover keys from the backup, a user will need to enter a recovery key to decrypt the backup. The backup will be encrypted using public key cryptography, so that any of a user's devices can back up keys without needing the user to enter the recovery key until they need to read from the backup.

See also:

• https://github.com/matrix-org/matrix-doc/issues/1219
• https://github.com/vector-im/riot-web/issues/3661
• https://github.com/vector-im/riot-web/issues/5675
• https://docs.google.com/document/d/1MOoIA9qEKIhUQ3UmKZG-loqA8e0BzgWKKlKRUGMynVc/edit# (old version of proposal)

Proposal

This proposal creates new APIs to allow clients to back up room decryption keys on the server. Room decryption keys are encrypted (using public key crypto) before being sent to the server along with some unencrypted metadata to allow the server to manage the backups. If a key for a new megolm session is uploaded, it is added to the current backup. If a key is uploaded for a megolm session is that is already present in the backup, the server will use the metadata to determine which version of the key is "better". The way in which the server determines which key is "better" is described in the Storing Keys section. The user is given a private recovery key in order to recover the keys from the backup in the future.

Clients can create new key backups (sometimes also referred to in the API as backup versions) to replace the current backup. Aside from the initial backup creation, a client might start a new a backup when, for example, a user loses a device and wants to ensure that that device does not get any new decryption keys. In this case, the client will then create a new backup using a new key that the device does not have access to.

Once one client has created a backup, other clients can fetch the public part of the recovery key from the server and add keys to the backup, if they trust that the backup was not created by a malicious device.

Possible UX for interactive clients

This section gives an example of how a client might handle key backups. Clients may behave differently.

On receipt of encryption keys (1st time):

1. client checks if there is an existing backup: GET /room_keys/version
   1. if not, ask if the user wants to back up keys
      1. if yes:
         1. generate new curve25519 key pair, which will be the recovery key
         2. create new backup: POST /room_keys/version
         3. display private key for user to save (see below for the format of the recovery key)
      2. if no, exit and remember decision (user can change their mind later)
      3. while prompting, continue to poll GET /room_keys/versions, as another device may have created a backup. If so, go to 1.2.
   2. if yes, either get the public part of the recovery key and check that it is signed by the master cross-signing key, or prompt user to enter the private part of the recovery key (which can derive the public part).
      1. User can also decide to create a new backup, in which case, go to 1.1.
2. send key to backup: PUT /room_keys/keys/${roomId}/${sessionId}?version=$v
3. continue backing up keys as we receive them (may receive a M_WRONG_ROOM_KEYS_VERSION error if a new backup has been created: see below)

On M_WRONG_ROOM_KEYS_VERSION error when trying to PUT keys:

1. get the current version
2. notify the user that there is a new backup, and display relevant information
3. confirm with user that they want to use the backup (user may want use the backup, to stop backing up keys, or to create a new backup)
4. ensure the public part of the recovery key is signed by the user's master key, or prompt the user to enter the private part of the recovery key

On receipt of undecryptable message:

1. ask user if they want to restore backup (ask whether to get individual key, room keys, or all keys). (This can be done in the same place as asking if the user wants to request keys from other devices.)
2. if yes, prompt for private key, and get keys: GET /room_keys/keys

Users can also set up, disable, or rotate backups, or restore from backup via user settings.

Recovery key

The recovery key can be saved by the user directly, stored encrypted on the server (using the method proposed in MSC1946), or both. If the key is saved directly by the user, then the code is constructed as follows:

1. The 256-bit curve25519 private key is prepended by the bytes 0x8B and 0x01
2. All the bytes in the string above, including the two header bytes, are XORed together to form a parity byte. This parity byte is appended to the byte string.
3. The byte string is encoded using base58, using the same mapping as is used for Bitcoin addresses.

This 58-character string is presented to the user to save. Implementations may add whitespace to the recovery key; adding a space every 4th character is recommended.

When reading in a recovery key, clients must disregard whitespace. Clients must base58-decode the code, ensure that the first two bytes of the decoded string are 0x8B and 0x01, ensure that XOR-ing all the bytes together results in 0, and ensure that the total length of the decoded string is 35 bytes. Clients must then remove the first two bytes and the last byte, and use the resulting string as the private key to decrypt backups.

Encoding the recovery key for server-side storage via MSC1946

If MSC1946 is used to store the key on the server, it must be stored using the account_data type m.megolm_backup.v1.

As a special case, if the recovery key is the same as the curve25519 key used for storing the key, then the contents of the m.megolm_backup.v1 account_data for that key will be an object with a passthrough property whose value is true. For example, if m.megolm_backup.v1 is set to:

{
  "encrypted": {
    "key_id": {
      "passthrough": true
    }
  }
}

means that the recovery key for the backup is the same as the private key for the key with ID key_id. (This is mostly intended to provide a migration path for for backups that were created using an earlier draft that stored the recovery information in the auth_data.)

API

Backup versions

POST /room_keys/version

Create a new backup version.

Body parameters:

• algorithm (string): Required. The algorithm used for storing backups. Currently, only m.megolm_backup.v1.curve25519-aes-sha2 is defined.
• auth_data (object): Required. algorithm-dependent data. For m.megolm_backup.v1.curve25519-aes-sha2, see below for the definition of this property.

Example:

{
  "algorithm": "m.megolm_backup.v1.curve25519-aes-sha2",
  "auth_data": {
    "public_key": "abcdefg",
    "signatures": {
      "something": {
        "ed25519:something": "hijklmnop"
      }
    }
  }
}

On success, returns a JSON object with keys:

• version (string): the backup version

GET /room_keys/version/{version}

Get information about the given version, or the current version if /{version} is omitted.

On success, returns a JSON object with keys:

• algorithm (string): Required. Same as in the body parameters for POST /room_keys/version.
• auth_data (object): Required. Same as in the body parameters for POST /room_keys/version.
• version (string): Required. The backup version.
• etag (string): Required. The etag value which is an opaque string representing stored keys in the backup. Clients can compare it with the etag value they received in the response of their last key storage request. If not equal, another client has pushed new keys to the backup.
• count (number): Required. The number of keys stored in the backup.

Error codes:

• M_NOT_FOUND: No backup version has been created. (with HTTP status code 404)

PUT /room_keys/version/{version}

Update information about the given version. Only auth_data can be updated.

Body parameters:

• algorithm (string): Required. Must be the same as in the body parameters for GET /room_keys/version.
• auth_data (object): Required. algorithm-dependent data. For m.megolm_backup.v1.curve25519-aes-sha2, see below for the definition of this property.
• version (string): Optional. The backup version. If present, must be the same as the path parameter.

Example:

{
  "algorithm": "m.megolm_backup.v1.curve25519-aes-sha2",
  "auth_data": {
    "public_key": "abcdefg",
    "signatures": {
      "something": {
        "ed25519:something": "hijklmnop"
        "ed25519:anotherthing": "abcdef"
      }
    }
  },
  "version": "42"
}

On success, returns the empty JSON object.

Error codes:

• M_NOT_FOUND: This backup version was not found. (with HTTP status code 404)

Storing keys

PUT /room_keys/keys/${roomId}/${sessionId}?version=$v

Store the key for the given session in the given room, using the given backup version.

If the server already has a backup in the backup version for the given session and room, then it will keep the "better" one. To determine which one is "better", keys are compared:

• first by the is_verified flag (true is better than false),
• then, if is_verified is equal, by the first_message_index (a lower number is better),
• and finally, is is_verified and first_message_index are equal, by forwarded_count (a lower number is better).

If neither key is better than the other (that is, if all three fields are equal), then the server should keep the existing key.

Body parameters:

• first_message_index (integer): Required. The index of the first message in the session that the key can decrypt.
• forwarded_count (integer): Required. The number of times this key has been forwarded.
• is_verified (boolean): Required. Whether the device backing up the key has verified the device that the key is from.
• session_data (object): Required. Algorithm-dependent data. For m.megolm_backup.v1.curve25519-aes-sha2, see below for the definition of this property.

On success, returns a JSON object with keys:

• etag (string): Required. The new etag value representing stored keys. See GET /room_keys/version/{version} for more details.
• count (number): Required. The new count of keys stored in the backup.

Error codes:

• M_WRONG_ROOM_KEYS_VERSION: the version specified does not match the current backup version (with HTTP status code 403). The current backup version will be included in the current_version field of the HTTP result.

Example:

PUT /room_keys/keys/!room_id:example.com/sessionid?version=1

{
  "first_message_index": 1,
  "forwarded_count": 0,
  "is_verified": true,
  "session_data": {
    "ephemeral": "base64+ephemeral+key",
    "ciphertext": "base64+ciphertext+of+JSON+data",
    "mac": "base64+mac+of+ciphertext"
  }
}

Result:

{
  "etag": "abcdefghi",
  "count": 10
}

PUT /room_keys/keys/${roomId}?version=$v

Store several keys for the given room, using the given backup version.

Behaves the same way as if the keys were added individually using PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Body parameters:

• sessions (object): an object where the keys are the session IDs, and the values are objects of the same form as the body in PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Returns the same as PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Example:

PUT /room_keys/keys/!room_id:example.com?version=1

{
  "sessions": {
    "sessionid": {
      "first_message_index": 1,
      "forwarded_count": 0,
      "is_verified": true,
      "session_data": {
        "ephemeral": "base64+ephemeral+key",
        "ciphertext": "base64+ciphertext+of+JSON+data",
        "mac": "base64+mac+of+ciphertext"
      }
    }
  }
}

Result:

{
  "etag": "abcdefghi",
  "count": 10
}

PUT /room_keys/keys?version=$v

Store several keys, using the given backup version.

Behaves the same way as if the keys were added individually using PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Body parameters:

• rooms (object): an object where the keys are the room IDs, and the values are objects of the same form as the body in PUT /room_keys/keys/${roomId}/?version=$v.

Returns the same as PUT /room_keys/keys/${roomId}/${sessionId}?version=$v

Example:

PUT /room_keys/keys/!room_id:example.com?version=1

{
  "rooms": {
    "!room_id:example.com": {
      "sessions": {
        "sessionid": {
          "first_message_index": 1,
          "forwarded_count": 0,
          "is_verified": true,
          "session_data": {
            "ephemeral": "base64+ephemeral+key",
            "ciphertext": "base64+ciphertext+of+JSON+data",
            "mac": "base64+mac+of+ciphertext"
          }
        }
      }
    }
  }
}

Result:

{
  "etag": "abcdefghi",
  "count": 10
}

Retrieving keys

When retrieving keys, the version parameter is optional, and defaults to retrieving keys from the latest backup version.

GET /room_keys/keys/${roomId}/${sessionId}?version=$v

Retrieve the key for the given session in the given room from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

Error codes:

• M_NOT_FOUND: The session is not present in the backup, or the requested backup version does not exist. (with HTTP status code 404)

GET /room_keys/keys/${roomId}?version=$v

Retrieve the all the keys for the given room from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys/${roomId}?version=$v.

If the backup version exists but no keys are found, then this endpoint returns a successful response with body:

{
  "sessions": {}
}

Error codes:

• M_NOT_FOUND: The requested backup version does not exist. (with HTTP status code 404)

GET /room_keys/keys?version=$v

Retrieve all the keys from the backup.

On success, returns a JSON object in the same form as the request body of PUT /room_keys/keys?version=$v.

If the backup version exists but no keys are found, then this endpoint returns a successful response with body:

{
  "rooms": {}
}

Error codes:

• M_NOT_FOUND: The requested backup version does not exist. (with HTTP status code 404)

Deleting keys

DELETE /room_keys/keys/${roomId}/${sessionId}?version=$v

DELETE /room_keys/keys/${roomId}?version=$v

DELETE /room_keys/keys/?version=$v

Deletes keys from the backup.

Returns the same as PUT /room_keys/keys/${roomId}/${sessionId}?version=$v.

m.megolm_backup.v1.curve25519-aes-sha2 definitions

auth_data for backup versions

The auth_data property for the backup versions endpoints for m.megolm_backup.v1.curve25519-aes-sha2 is a signed json object with the following keys:

• public_key (string): the curve25519 public key used to encrypt the backups
• signatures (object): signatures of the auth_data.

The auth_data should be signed by the user's master cross-signing key, and may also be signed by the user's device key. This allows clients to ensure that the public key is valid, and prevents an attacker from being able to change the backup to use a public key that they have the private key for.

session_data for key backups

The session_data field in the backups is constructed as follows:

1. Encode the session key to be backed up as a JSON object with the properties:
   • algorithm (string): m.megolm.v1.aes-sha2
   • sender_key (string): base64-encoded device curve25519 key
   • sender_claimed_keys (object): object containing the identity keys for the sending device
   • forwarding_curve25519_key_chain (array): zero or more curve25519 keys for devices who forwarded the session key
   • session_key (string): base64-encoded (unpadded) session key in session-sharing format
2. Generate an ephemeral curve25519 key, and perform an ECDH with the ephemeral key and the backup's public key to generate a shared secret. The public half of the ephemeral key, encoded using base64, becomes the ephemeral property of the session_data.
3. Using the shared secret, generate 80 bytes by performing an HKDF using SHA-256 as the hash, with a salt of 32 bytes of 0, and with the empty string as the info. The first 32 bytes are used as the AES key, the next 32 bytes are used as the MAC key, and the last 16 bytes are used as the AES initialization vector.
4. Stringify the JSON object, and encrypt it using AES-CBC-256 with PKCS#7 padding. This encrypted data, encoded using base64, becomes the ciphertext property of the session_data.
5. Pass the raw encrypted data (prior to base64 encoding) through HMAC-SHA-256 using the MAC key generated above. The first 8 bytes of the resulting MAC are base64-encoded, and become the mac property of the session_data.

(The key HKDF, AES, and HMAC steps are the same as what are used for encryption in olm and megolm.)

Security Considerations

An attacker who gains access to a user's account can delete or corrupt their key backup. This proposal does not attempt to protect against that.

An attacker who gains access to a user's account can create a new backup version using a key that they control. For this reason, clients SHOULD confirm with users before sending keys to a new backup version or verify that it was created by a trusted device by checking the signature. Alternatively, if the signature cannot be verified, the backup can be validated by prompting the user to enter the recovery key, and confirming that the backup's public key corresponds to the recovery key.

Other Issues

Since many clients will receive encryption keys at around the same time, they will all want to back up their copies of the keys at around the same time, which may increase load on the server if this happens in a big room. (TODO: how much of an issue is this?) For this reason, clients should offset their backup requests randomly.

Conclusion

This proposal allows users to securely and conveniently back up and restore their decryption keys so that users logging into a new device can decrypt old messages.